Fire (or combustion) is a chemical reaction in which a substance combines with oxygen and heat is released. Usually, fire occurs when a source of heat comes into contact with a combustible material. If a combustible liquid or solid is heated it evolves vapor and if the concentration of vapor is high enough, it forms a flammable mixture with oxygen in the air. If this flammable mixture is then heated further to its ignition point, combustion starts.
Reference: Lees’ Loss Prevention in the Process Industries
Technical Tools/Risk Assessment/Quantitative Risk Assessment/Consequence Techniques